1. Field of the Invention
The present invention relates to a diagnostic apparatus for a dynamic system, and more particularly, to an apparatus for very accurately diagnosing faults in a dynamic system, the air pressure of a tire, and conditions around the tire by vibrating the dynamic system through use of vibration means provided in the diagnostic apparatus and on the basis of internal disturbance caused by a fault in the thus-vibrated dynamic system even in a case where the amount of external disturbance is small.
Further, the present invention relates to a braking pressure estimation device which estimates master cylinder pressure, mean braking pressure, and a micro-vibration component in the braking pressure on the basis of wheel speed information and without use of a braking pressure sensor, by taking note of the gain characteristics of a wheel speed and the braking pressure in a case where the braking pressure is minutely vibrated at a resonance frequency of a vibration system comprised of a vehicle body, a wheel, and a road surface. Still further, the present invention relates to an anti-lock brake controller which employs the braking pressure estimation device, as well as to a braking pressure controller capable of continually controlling braking pressure in a smooth manner.
2. Related Art
(Conventional Apparatus for Diagnosing a Dynamic System)
Japanese Patent Application Laid-open (JP-A) No. 7-98268 describes the following technique as an apparatus for diagnosing a dynamic system.
In the technique disclosed in the foregoing unexamined patent application, a total disturbance vector--which is the sum of an external disturbance vector exerted on a dynamic system from outside and an internal disturbance vector caused by a fault in the dynamic system--is estimated on the basis of a response output vector from the dynamic system. Further, an internal state quantity vector of the dynamic system is estimated. A component relevant to internal disturbance is separated from the total disturbance vector by calculation of the correlation between the thus-estimated total disturbance vector and the internal state quantity vector. Then, a part of the dynamic system corresponding to the thus-separated component relevant to the internal disturbance is specified as a fault.
The foregoing technique enables location of an area of the dynamic system to be examined through use of one observer, as well as estimation of the absence or presence of a fault in the area and the degree of the fault. As contrasted with a conventional generalized likelihood comparison test method, there is no need for this technique to examine a plurality of areas through use of a plurality of observers disposed so as to correspond to the areas, and hence the technique has an advantage of being capable of accurately sensing a fault with few calculations.
(Conventional Braking Pressure Estimation Device, and Anti-lock Brake Controller Utilizing the Braking Pressure Estimation Device)
With regard to a device which controls the braking force of wheels such as an anti-lock brake controller (hereinafter referred to as an ABS controller), the sensing of braking pressure is an important problem, and a higher degree of control of the braking pressure can be expected if the value of the braking pressure is known. As a method for sensing the braking pressure, there is a method of attaching pressure sensors comprising a semiconductor device or the like to each wheel cylinder. However, the pressure sensors are comparatively expensive, and attachment of the pressure sensors to each wheel cylinders is difficult in terms of cost.
To overcome such a difficulty, there has already been proposed a technique of estimating the wheel cylinder pressure of each wheel on the basis of the result of measurement of the pressure sensor attached to the master cylinder pressure. Japanese Patent Application Laid-open (JP-A) No. 7-186918 describes a braking pressure controller which employs a method of calculating the wheel cylinder pressure of each wheel from a measured supply pressure (e.g., master cylinder pressure) and the time period over which a valve is actuated. This method enables the estimation of the wheel cylinder pressure of each wheel solely by sensing the pressure of the master cylinder through use of one pressure sensor, thereby rendering the braking pressure controller advantageous in terms of cost.
However, in many widely-available ABS controllers, a pressure sensor is not attached even to the master cylinder. Therefore, even the foregoing wheel cylinder estimation technique that requires the attachment of a pressure sensor to only the master cylinder still adds to the cost. Further, there is needed a remedy for sensor failures in order to ensure the reliability of the pressure sensor, which also adds to the cost.
Japanese Patent Application Laid-open (JP-A) No. 6-286590 describes a method of estimating master cylinder pressure without use of a pressure sensor.
The technique described in the foregoing patent application discloses the following three methods for estimating master cylinder pressure.
(1) Assuming that there is a uniform increase in the master cylinder pressure as a result of a first reduction in hydraulic braking pressure, a variation .DELTA.Pm (constant) in the master cylinder pressure is calculated. The master cylinder pressure is corrected through use of the thus--calculated variation .DELTA.Pm.
(2) By utilization of a tendency for the variation in the master cylinder pressure to increase in accordance with a variation .DELTA.S in slip rate so long as the value of the master cylinder pressure is large, the variation .DELTA.Pm in the master cylinder pressure is calculated from the variation in the slip rate. Accordingly, as contrasted with the method described in (1) which is based on the assumption that the variation in the master cylinder pressure is constant, degree of accuracy of estimation is improved.
(3) The variation .DELTA.Pm in the master cylinder is calculated from a variation .DELTA.V'.sub.w0 in vehicle acceleration and the variation .DELTA.S in the slip rate. The use of the variation .DELTA.V'.sub.w0 in vehicle acceleration enables correction of the master cylinder pressure, even when slippage of wheels due to speed reduction has not yet occurred and therefore the master cylinder pressure Pm is low. Accordingly, as contrasted with the method described in (2), this method further improves the accuracy of estimation of master cylinder pressure.
(Conventional Hydraulic Braking Pressure Controller)
For an apparatus such as an ABS controller in which the braking force applied to the vehicle is controlled by the hydraulic pressure of the wheel cylinder, not only a reduction in the hydraulic braking pressure at the time of wheel lock, but also smooth continuous control of the hydraulic braking pressure is important.
The hydraulic braking pressure is controlled by pressure increase and decrease valves (such as those disposed in an ABS actuator shown in FIGS. 23A and 23B) that are controllable from outside by means of an electrical signal, or the like, and are interposed between the master cylinder--whose pressure is related to the depression of a brake pedal given by the driver--and the wheel cylinders which produce pressure for pressing brake pads; i.e., the actual braking force of the vehicle.
Under normal conditions, the pressure in the master cylinder is directly transmitted to the hydraulic pressure in the wheel cylinders by opening the pressure increase valves of the ABS actuator shown in FIGS. 23A and 23B, as well as by closing the pressure decrease valves of the same. If the hydraulic pressure in the wheel cylinders is excessively high, there is established a state (a pressure decrease mode) in which the pressure increase valve is closed and the pressure decrease valve is opened so that the pressure from the master cylinder is prevented from being transmitted to the wheel cylinders while the hydraulic pressure in the wheel cylinders is released to a reservoir tank. In order to allow the hydraulic pressure in the wheel cylinders to recover from the pressure-reduced state, there is established a state (a pressure hold mode) in which both the pressure increasing and reducing valves are closed to prevent the hydraulic pressure in the wheel cylinders from changing; a state (a pressure increase mode) in which only the pressure increase valve is opened to allow the master cylinder pressure to be directly transmitted to the wheel cylinders is inserted over a short period of time; and the time period of the pressure increase mode is gradually increased.
In an actual ABS control operation, if wheel lock is detected, the hydraulic pressure in the wheel cylinders is abruptly reduced through the pressure decrease mode which is continued for a comparatively long period of time. Subsequently, the hydraulic pressure is rather gradually increased by repeating the pressure hold mode and pressure increase mode. If wheel lock arises again during the course of an increase in the hydraulic pressure, the valve is controlled in the pressure decrease mode to thereby abruptly decrease the hydraulic pressure in the wheel cylinders. Subsequently, the operation mode is shifted to the pressure hold mode and the pressure increase mode.
For example, in the technique disclosed in Japanese Patent Application Laid-open (JP-A) No. 3-118263, for each pressure increase and reduction area, the pulse train pattern is switched through use of a plurality of maps as well as the relationship between wheel speed, acceleration/deceleration, and separately-determined target speed, as shown in FIGS. 24A and 24B.
Further, Japanese Patent Application Laid-open (JP-A) No. 8-34329 describes a technique of controlling mean hydraulic braking pressure on the basis of the operation frequency of a valve and a duty ratio between a pressure increase mode and a pressure decrease mode through use of a valve having only a pressure increase mode and a pressure decrease mode. This technique principally implements the pressure hold mode of a hydraulic braking pressure by actuating a valve at a comparatively high frequency through use of only the valve having a pressure increase mode and a pressure decrease mode. Further, the amplitude of micro-vibrations in the hydraulic braking pressure is controlled by changing the operation frequency of the valve.
In the foregoing technique described in Japanese Patent Application Laid-open (JP-A) No. 7-98268, a total disturbance vector of a dynamic system is estimated on the basis of a response output from the dynamic system stemming from the disturbance outside the dynamic system. For example, in a dynamic system comprising a suspension system and a wheel, the external disturbance exerted on wheels traveling on an a rough road surface is utilized as an input. Accordingly, in a case where the input stemming from the external disturbance is small such as that obtained when the wheels travel on a smooth road surface, the response output becomes small. Therefore, even if the output value is normalized, the accuracy of sensing of a fault or the accuracy of estimation of air pressure in the tire is reduced by a quantization error or the like.
In a case where the mean value of external disturbance obtained during a given period of time is zero or nearly zero, the foregoing conventional technique enables highly accurate separation of the component which is relevant to an internal disturbance vector and stems from a fault from a total disturbance vector. However, the statistical property of the external disturbance differs according to the state of a road surface, and sometimes the internal disturbance vector cannot be very accurately isolated.
Further, even in the case of the method (3) having the highest degree of accuracy among the techniques described in Japanese Patent Application (JP-A) No. 6-286590, since the method employs the variation .DELTA.S in the slip rate, the relationship between the variation .DELTA.S in the slip rate and the variation .DELTA.Pm in the master cylinder pressure changes as a result of a variation in the coefficient of friction .mu. between the tire and the road surface. The calculation of the variation .DELTA.Pm in the master cylinder pressure through use of a constant coefficient results in a reduction in the accuracy of estimation according to the state of the road surface.
In the conventional technique described in Japanese Patent Application Laid-open (JP-A) No. 3-118263, the hydraulic braking pressure is controlled by switching the pulse train pattern between the patterns shown in FIGS. 24A and 24B. For this reason, as shown in FIG. 24C, a vibration arises in the hydraulic pressure in the wheel cylinders at a comparatively low frequency (several hertz). This vibration imparts to the driver's pedal an unpleasant low frequency vibration called kickback and results in a great variation in vehicle behavior at a comparatively low frequency. As a result, the hydraulic braking pressure cannot be controlled smoothly.
The technique described in Japanese Patent Application Laid-open (JP-A) No. 8-34329 cannot be applied to the anti-lock brake control operation described in Japanese Patent Application Laid-open (JP-A) No. 7-220920 which utilizes the fact that the resonance characteristics of the resonance system comprised of a vehicle body, a wheel, and a road surface change according to the state of grip of the wheel on the road surface, because it is impossible to change the frequency of vibration at the resonance frequency of the system.